Configuring Digital Input Filter Times - Siemens SIMATIC S7-1200 System Manual

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Device configuration

6.6 Configuring the operation of the CPU

Configuring digital input filter times

The digital input filters protect your program from responding to unwanted fast changes in

the input signals, as may result from switch contact bounce or electrical noise. The default

filter time of 6.4 ms blocks unwanted transitions from typical mechanical contacts. Different

points in your application can require shorter filter times to detect and respond to inputs from

fast sensors, or longer filter times to block slow contact bounce or longer impulse noise.

An input filter time of 6.4 ms means that a single signal change, from '0' to '1' or from '1' to

'0', must continue for approximately 6.4 ms to be detected, and a single high or low pulse

shorter than approximately 6.4 ms is not detected. If an input signal switches between '0'

and '1' more rapidly than the filter time, the input point value can change in the user program

when the accumulated duration of new value pulses over old value pulses exceeds the filter

The digital input filter works this way:

● When a "1" is input, it counts up, stopping at the filter time. The image register point

changes from "0" to "1" when the count reaches the filter time.

● When a "0" is input, it counts down, stopping at "0". The image register point changes

from "1" to "0" when the count reaches "0".

● If the input is changing back and forth, the counter will count up some and count down

some. The image register will change when the net accumulation of counts reaches

either the filter time or "0".

● A rapidly-changing signal with more "0's" than "1's" will eventually go to "0", and if there

are more "1's" than "0's", the image register will eventually change to "1".

Each input point has a single filter configuration that applies to all uses: process inputs,

interrupts, pulse catch, and HSC inputs. To configure input filter times, select "Digital Inputs".

S7-1200 Programmable controller

System Manual, V4.2, 09/2016, A5E02486680-AK